R in wash liquor



Jn. 31, 1956 A. s. GRIFFITH, JR., ETI'AL 2,733,121

PRESSURE WASHING PROCESS Filed June 8, 1951 Fl G. I.

40 50 WASH TEMPERATURE 2 c Q Q Q o o m h m If) v- 10 N D.M.F. IN WASH LIQUOR 300 350 400 SPINNING SPEED Y.P.M.

FIG. 11.

INVENTORS: WASH L'QUOR Andrew Starling Griff/'fh,J/'.,

Robert Edwin McDonald 8 BY Robert Edward Wi/fong A TTORNE Y United States Patent "ice PRESSURE WASHING PROCESS Andrew Starling Griffith, Jr., and Robert Edwin McDonald, Waynesboro, and Robert Edward Wilfong, Staunton, Va., assignors to E. I. du Pont de lflemours z Company, wilmington belifacorpo'fation of Delaware Application June 8, 1951, Serial No. 230,572

Claims. (Cl. 1854) This invention relates to a process for preparing acrylonitrile polymer yarns containing a small controlled amount of solvent and in particular to a process for pressure washing acrylonitrile polymer yarns, which have been dry-spun from solutions of the polymer in dimethylformamide. Still more particularly it relates to stretching specially washed yarns.

Polyacrylonitrile and copolymersof acrylonitrile with other polymerizable substances, for. example, vinyl or acrylic compounds in which at least 85% by weight of the polymer is acrylonitrile, have been known for some time and recognized as possessing desirable physical and chemical properties, including toughness and insolubility in and insensitivity to common organic solvents. Continuous filament yarns are conveniently prepared from organic solvent solutions of the acrylonitrile polymer by a dry-spinning technique. Most of the synthetic organic polymer yarns are prepared from polymeric materials which are soluble in lowboiling solvents such as acetone, and it is relatively easy to remove the solvent medium by evaporation. However, acrylonitrile polymers which contain at least 85% by weight of acrylonitrile are insoluble in low boiling solvents and special solvents, such as those disclosed in U. S. 2,404,714 to U. S. 2,404,727 inclusive, are necessary. The dry-spinning of such solutions requires much higher temperature than normally used in dry-spinning techniques in order to remove enough of the solvent by evaporation so that a non-tacky yarn can be collected on a suitable winding device. Since undesirable color formation is attendant with the exposure of polymers and solutions of polymers to high temperatures, it is desirable to restrict the time of exposure of the freshly spun filaments to the hot evaporative medium. Accordingly, in the dry-spinning of yarns from acrylonitrile polymers dissolved, for example, in dimethylformamide, only part of the solvent is removed by the evaporative process, and the yarn collected from the spinning cell may contain from 15-40% or more by weight of dimethylformamide.

This solvent-laden yarn has poor tensile properties and correspondingly low commercial utility. The majority of the residual solvent must be removed prior to drawing and its removal must lead to a substantially constant concentration in order to produce commercial yarns of high strength and utility.

The removal of solvent by fresh water washing is not attractive for several reasons. It is impracticable to control time and temperature of such washings so that the washed products have uniform concentrations of solvent. Uniformity of solvent content of yarn to be drawn is extremely important, for both drawing continuity and uniformity of the final product vary with the amount of residual solvent. To assure the production of yarn which does not vary greatly in its physical properties along the 2,733,121 Patented Jan. 31, 1956 5 by weight of the yarn.

It is an object of this invention to provide an undrawn polymer yarn having a substantially constant solvent con- -tent..of.betw.een1.0% and 2.5%. Another object of this invention is to provide a process for reducing the solvent content of dry-spun acrylonitrile polymer yarn to this predetermined low level prior to the drawing operation. It is a further object of this invention to provide a process for washing solvent-laden, dry-spun acrylonitrile polymer yarn to a uniformly predetermined low level of solvent content. It is a particular object of this invention to provide a washing process for -reducing the solvent content of acrylonitrile polymer yarn dry-spun from solutions in dimethylformamide to a level in the range of from about 1.0% to about 2. 5%, by weight of dimethylforrnamide. Still another object is to provide a process which reproducibly leaves the small amount of residual dimethylformamide distributed uniformly throughout the the yarn. Other objects become obvious from the description of the invention which follow.

, The objects of thisfrnvention are-..accomplished ,by

, means of a process whiclicomprijes .presirrewashing packages of acrylonitrile-polymer yarn containing from to 40% solvent with dilute. aqueous solutions of the solvent to a substantially constant low solvent content. The aqueous solutions of the solvents are forced through the package at rates of 'fromabout 0.2 to about 1.2 gals./ min. and the resultant washed yarn has 'a solvent content uniformly distributed throughout the yarn and has a value between about 1.0% and about 2.5%. This yarn of uniform low: solvent'content is then stretched. Referring specifically to polyacrylonitrile yarn spun from dimethylformamaide solutions, the yarn containing from 15% to 40% dimethylformamide is washed with a dilute aqueous solution of dimethylformamide at temperatures in the range 105 C. to essentially the equilibrium point, that is, the time after which further washing with the aqueous dimethylformamide solution does not change thesolvent content of the yarn. By pressure washing packages of yarn is meant the forced passage of the wash liquor through the yarn wound in the form of cakes or on perforated bobbins, pirns or the'like. The wash liquors may be applied at their boiling points which will usually be above 100 C. as, for example, about 105 C. for certain aqueous dimethylformamide solutions.

The following examples are illustrative but not limitative of the invention:

EXAMPLE I A series of experiments was performed in which ,350 denier, 20 filament yarn and 700 denier, 40 filament yarn were dry spun from polyacrylonitrile dissolved in dimethylformamide and collected by means of a down twister type take-up on large perforated aluminum pirns at the rate of 300 y. p. m.

- The polyacrylonitrile yarns thus collected contained from 15-40 percent or more by weight of dimethylformamide. The several pirns of yarn were then washed for 1 hour at various temperatures with dilute aqueous solutions of dimethylformamide. The wash liquor was introduced into the pirn barrel and forced outwardly through the yarn package under pressures ranging from -110 p. s. i. or to give a How of about 0.7 gaL/min. per package. The dimethylformamide contents of the resulting yarns are shown in Table I.

Table I Percent by Percent by Weight Weight Wash T, C. DMF in DMF in Wash Dry Liquor Yarn From the above results it can be seen that surprisingly higher concentrations of dimethylformaxnide in the wash liquor are needed for the desired solvent concentration in the treated yarn when operating at relatively high wash liquor temperatures.

Using the equation described hereinafter the above experiments were repeated in part using the wash liquor concentrations calculated to leave 1.75% by weight of dimethylformamide in the dry yarn. At temperatures of 57, 60 and 90 C. wash liquors containing 2.3%, 2.1%, 2.3% and 11.0% dimethylformam'rde, respectively were used. In each case the washed and dried yarn contained 1.75 $0.17 by weight of dimethylformamide.

EXAMPLE II The effect of spinning speed on the wash liquor concentration required to leave a specific concentration of dirnethylformamide in the washed yarn is illustrated by the following series of experiments. Polyacrylonitrile yarns dry spun from dimethylformamide solution were collected on perforated pirns at speeds in the range of 120-380 yards per minute. The solvent content of the spun yarns ranged from 20-45% by weight on a dry basis. These yarn packages were pressure washed to give a flow rate of about 0.8 gal./min. per package for 45 minutes using 98 C. wash liquor at the concentrations shown in Table II.

In each case, it was found that the washed yarn contained l.75i0.17% by weight of dimethylformarnide on a dry basis and yet the concentration of dimethylformamide in the wash liquor was 2% times higher for yarn spun at the highest spinning speed as compared with the yarn spun at the lowest speed. This wide variation in wash liquor concentration is indeed surprising but empirical correction for temperature and yarn spinning speed as developed in the formula that follows has been most helpful in selecting effective washing conditions.

The results of the experiments of Examples I and II are plotted in Figures I and II using those experiments which resulted in polyacrylonitrile yarn containing 1.75:0.17% by weight of residual dimethyl formamide (dry yarn basis).

An equation relating yarn spinning speed and wash liquor temperature with the weight percent of dimethylformamide to be used in the aqueous wash liquor has been derived from similar data to that given above. The con.- centration of the dimethylformamide. to be employed in the aqueous wash liquor depends upon the speed at which the solvent-laden yarn was spun and on the temperature 4 of the wash liquor, and it may be determined from a formula of the type ou vas) X T T7? wherein C= percent by weight of dimethyltormamide in the aqueous wash liquor, S=spinning speed in yards per minute, T=a function of the temperature of the wash liquor in degrees centigrade, A and B are constants, and X=the desired specific residual solvent concentration in "'fihehgashed yarn in the range of 1.0-2.5% dimethylformequation the upstream. pressure;

amidefThTsfieclfic-equafionwhigorrespoms to the curves of Figures I and II, wherein e content of the yarn after washing is 1.75 :0. 17% by weight takes the following form:

C gtossumomes) X where T=5.4 between 20 and 60., and where T= l0 1.45 between 60 and 100 C., wherein C=percent by weight of dimethylformarnide in the aqueous wash liquor, S= spinning speed in yards per minute, =the temperature of the wash liquor in degrees centigrade, and X==the desired percent by weight of dimethylformamide in the washed yarn (dry basis). This correlates these effects to an accuracy of 110% of the calculated value for the solvent concentration in the .wash liquor, in all variations of spinning speed, spun yarn solvent content, or liquor temperature.

The washingpressure to be used depends chiefly on the package structure, size and hardness and the flow rate desired. At very high flow rates, the wash liquor has a tendency to channel its way through the yarn package and the yarn is not uniformly washed. High flow rates may also cause considerable damage to the yarn packages. At very low flow rates the package distorts resulting in poor washing uniformity and subsequent processing difliculties. The most satisfactory method for controlling the how rate is to install a flow limiting orifice in each wash chuck position. The size of the orifice depends on the pressure drop thru the package and theflow. rate desired. In general, upstream pressures in the range -150 pounds per square inch giving flow rates from 0.2 to 1.5 gals/min. per package will be used. Thisrate should preferably be maintained at about 0.5 to 0.8 gaL/min. i 1

Better control of extent anduniformity of dimethyiformamide in the washed yarn is obtained by the process of this invention than has been possible heretofore because washing times in excess of those required to reduce the solvent content in the yarn to the desired level may be employed without substantially changing this solvent content.

The wash liquor will contain about 2% to about 15% ot the solvent, the extracting liquor concentration ior a specific solvent being'chosen to leave a residual solvent level in the yarn of about 1.0% to about 2.5 -72: by weight (dry yarn basis.) in 15 to 120 minutes washing time. Preferably and at the higher temperatures this time can be kept below about 75 minutes, and for good uniformity the time should not be shortened below about 50 minutes. The concentration of from 1.0% to 2.5% of residual solvent in the washed and dried yarn was chosen because of improvement in drawing continuity and final yarn physical properties. Below 1% residual solvent, continuity of drawing is poor and the physical properties are the solvent level desired in the extracted yarn may be obtained over the wide temperature range of from 20 C. to the boilingpoint of the wash liquor, at desirable speeds oi washing a preferred temperature range is from about C. to about C.

The process of this invention is applicable to acrylonitrile polymers containing at least 85% acrylonitrile. While the invention has been described with particular reference to polyacrylonitrile, copolymers may be similarly processed. Such copolymers include polymers containing styrene, methacrylic esters, vinyl pyridine, such as 2-vinyl pyridine, vinyl acetate, vinyl chloride or vinylidene chloride among others. Similarly, other solvents than dimethylformamide may be involved and included among these are those shown in U. S. Patents 2,404,714-2,404,- 727, N,N-dimethylacetamide, maleic anhydride, succinic anhydride, etc. Of course, the concentrations of the aqueous solutions of the solvents used in the washing step will depend, as will the time and temperature, upon the particular polymer being processed and upon the residual solvent. The conditions prevailing, as described above, for polyacrylonitrile yarn obtained using dimethylformamide are generally applicable.

In the interest of economic recovery of solvent from the wash liquor, it is advisable to divert the eflluent wash liquor from the first few minutes of washing to the recovery unit while the remainder is used, after suitable dilution, for further washing. The wash liquor from the first few minutes of the wash cycle is very rich in dimethylformamide and recovery is most economical from concentrated solutions.

The pressure washing process of this invention provides a means for reducing the solvent content of polyacrylonitrile yarn dry-spun from solutions of the polymer, as from dimethylformamide, to a desired uniform level in the range of from 12.5% by weight of the dry yarn. The residual dimethylformamide is distributed uniformly throughout the yarn so that the continuity of the subsequent drawing process is improved and the final yarn physical properties are uniform and more nearly optimum.

Any departure from the procedure described herein which conforms to the principles of the invention is intended to be included within the scope of the claims below.

We claim:

1. A process for the production of acrylonitrile polymer yarn containing a small amount of water-soluble, organic solvent uniform in amount and distribution which comprises spinning said yarn from a solution of said polymer in said solvent; winding said yarn on a perforated core; forcing through the resultant yarn package an aqueous wash liquor containing from about 2% to about 15% of said solvent, said liquor being under sufiicient pressure to cause a flow rate of about 0.2 to about 1.5 gallons per minute to produce thereby a yarn containing uniformly distributed therein a uniform amount of residual solvent being between about 1% and about 2.5%.

2. A process for the production of acrylonitrile polymer yarn containing from about 1% to about 2.5% of a water-soluble, organic solvent uniform in amount and distribution which comprises pressure washing acrylonitrile polymer yarn in package form and which contains a substantial amount of said solvent with an aqueous solution containing from about 2% to about 15% of said solvent at a flow rate of about 0.2 to about 1.5 gallons per minute per package for a period of about 15 to about 120 minutes.

3. A process in accordance with claim 2 wherein said polymer is derived from at least about by weight, of acrylonitrile.

4. A process for the production of acrylonitrile polymer yarn containing from about 1% to about 2.5% of a water-soluble organic solvent which comprises washing acrylonitrile polymer yarn that contains from about 15% to about 40% of said solvent with an aqueous solution containing from about 2% to about 15% of said solvent, said washing being continued substantially to the equilibrium point between the solvent content of said yarn and the solvent concentration of said solution, the equilibrium solvent content of said yarn being from about 1% to about 2.5 by weight.

5. A process for the production of acrylonitrile polymer yarn containing from about 1% to about 2.5 of a water-soluble, organic solvent uniform in amount and distribution which comprises pressure washing acrylonitrile polymer yarn in package form and which contains a substantial amount of said solvent with an aqueous solution containing from about 2% to about 15 of said solvent for a period of about 15 to about minutes.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR THE PRODUCTION OF ACRYLONITRILE POLYMER YARN CONTAINING A SMALL AMOUNT OF WATER-SOLUBLE, ORGANIC SOLVENT UNIFORM IN AMOUNT AND DISTRIBUTION WHICH COMPRISES SPINNING SAID YARN FROM A SOLUTION OF SAID POLYMER IN SAID SOLVENT; WINDING SAID YARN ON A PERFORATED CORE; FORCING THROUGH THE RESULTANT YARN PACKAGE AN AQUEOUS WASH LIQUOR CONTAINING FROM ABOUT 2% TO ABOUT 15% OF SAID SOLVENT, SAID LIQUOR BEING UNDER SUFFICIENT PRESSURE TO CAUSE A FLOW RATE OF ABOUT 0.2 TO ABOUT 1.5 GALLONS PER MINUTE TO PRODUCE THEREBY A YARN CONTAINING UNIFORMLY DISTRIBUTED THEREIN A UNIFORM AMOUNT OF RESIDUAL SOLVENT BEING BETWEEN ABOUT 1% AND ABOUT 2.5%. 